Be careful or you will loose the elastic. It is looped around the leader line, inside of the bar end. Attach an old piece of kite line to the end of the broken leader, and then pull the broken line through the bar end. Leave the kite line in place, and use it as a "pull" line to pull through the new leader line.

If the elastic has already come out of its hole, then be creative in repositioning it, before drawing the new leader line through the holes in the bar end. Use a piece of wire with a piece of old kiteline taped to the end of the wire to work with.